In continuous ink jet (CIJ) printing, the ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. The approach for printing with these droplet streams is to selectively charge and deflect certain drops from their normal trajectories. Graphic reproduction is accomplished by selectively charging and deflecting drops from the drop streams and depositing at least some of the drops on a print receiving medium while other drops strike a drop catcher device. The continuous stream ink jet printing process is described, for example, in U.S. Pat. Nos. 4,255,754, 4,698,123, and 4,751,517, the disclosures of each of which are totally incorporated herein by reference.
Generally, an ink for use in a CU system is required to have high waterfastness (reduced tendency to wick or bleed on a substrate when subjected to moisture) with excellent optical density and color tone, and yet be sufficiently fluid as to allow jetting through the nozzle. In the ink jet printing art, inks containing the water soluble dyes typically employed have the desired viscosity for runnability but are not considered to have a high degree of waterfastness. Typically, the solubility of the dye and the degree of ink waterfastness are inversely related. Polymers and resins may be added to the ink solution to enhance adhesion to the substrate and increase waterfastness. However, these polymers and resins also have the adverse affect of creating deposits on the printhead orifice and charge plates, adversely affecting machine runnability by increasing viscosity, and sufficiently high levels may have limited solubility.
Increasing the dye concentration in the ink in order to improve the optical density is limited by the solubility of the dye. If the solubility limit of the dye is exceeded the dye tends to crystallize out onto the orifice plate, causing crooked jets and poor runnability. Furthermore, the optical density of colors in general tends to level off at certain concentrations and adding more dye will have no effect on improving the optical density.
Polyethyleneimine (PEI) is often employed in ink compositions to increase waterfastness; however, PEI may decrease the solubility of the dye or worse, degrade the dye (reduction of the azo-linkage, which is the chemical group responsible for the color of the ink by the primary amino-group functional groups of PEI). Alkylation or hydroxyalkylation of the primary amino-group of branched polyethyleneimine has been suggested to decrease the ability of PEI to reduce the azo-linkage and preserve the dye. Unfortunately, as the percentage of alkylated/hydroxyalkylated primary and secondary amino-groups increase, the polymer becomes less soluble in water and precipitates out.
U.S. Pat. No. 5,017,644 discloses ink jet ink compositions which contain linear N-hydroxyethyl substituted polyethyleneimine polymers, and teaches using branched polymers gives results which are inferior in terms of waterfastness and lightfastness to those using linear polymers. Unfortunately, linear polymers are not commercially available, since the normal polymerization product of ethyleneimine has random side branches, which upon ethoxylation gives branched N-hydroxyethyl polyethyleneimine.
It is seen that there is still a need for improved compositions for coating or printing on a substrate, which would enable improved waterfast dye images printed with ink jet recording equipment.